On Mon, 16 Jul 2012, Bob Friesenhahn wrote:
On Mon, 16 Jul 2012, Stefan Ring wrote:
It is normal for reads from mirrors to be faster than for a single disk
because reads can be scheduled from either disk, with different I/Os being
handled in parallel.
That assumes that there *are* outstanding requests to be scheduled in
parallel, which would only happen with multiple readers or a large
read-ahead buffer.
That is true. Zfs tries to detect the case of sequential reads and requests
to read more data than the application has already requested. In this case
the data may be prefetched from the other disk before the application has
requested it.
This is my understanding of zfs: it should load balance read requests even
for a single sequential reader. zfs_prefetch_disable is the default 0. And
I can see exactly this scaling behaviour with sas disks and with scsi
disks, just not on this sata pool.
zfs_vdev_max_pending is already tuned down to 3 as recommended for sata
disks, iostat -Mxnz 2 looks something like
r/s w/s Mr/s Mw/s wait actv wsvc_t asvc_t %w %b device
507.1 0.0 63.4 0.0 0.0 2.9 0.0 5.8 1 99 c13t5d0
477.6 0.0 59.7 0.0 0.0 2.8 0.0 5.8 1 94 c13t4d0
when reading from the zfs mirror. The default zfs_vdev_max_pending=10
leads to much higher service times in the 20-30msec range, throughput
remains roughly the same.
I can read from the dsk or rdsk devices in parallel with real platter
speeds:
dd if=/dev/dsk/c13t4d0s0 of=/dev/null bs=1024k count=8192 &
dd if=/dev/dsk/c13t5d0s0 of=/dev/null bs=1024k count=8192 &
extended device statistics
r/s w/s Mr/s Mw/s wait actv wsvc_t asvc_t %w %b device
2467.5 0.0 134.9 0.0 0.0 0.9 0.0 0.4 1 87 c13t5d0
2546.5 0.0 139.3 0.0 0.0 0.8 0.0 0.3 1 84 c13t4d0
So I think there is no problem with the disks.
Maybe it's a corner case which doesn't matter in real world applications?
The random seek values in my bonnie output show the expected performance
boost when going from one disk to a mirrored configuration. It's just the
sequential read/write case, that's different for sata and sas disks.
Michael
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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